Elemental analysis, which generally refers to the process by which a sample (e.g., soil, waste or drinking water, bodily fluids, minerals, etc.) is analyzed for its elemental composition is central to a variety of analytical techniques that find use in, for example, environmental and medical applications.
Many metals or metalloids, for example, play important roles in physiological processes. The human body is exposed to various elemental forms of these metals or metalloids which are ubiquitous in animals, plants, rocks, soil, water, and air. Exposure to these metals or metalloids can be from air inhalation, food and water intake, surgical implants, and occupational scenarios, among others.
Selenium, a metalloid, is among the rarest elements on the surface of the planet and it is released through both natural and human activities. For example, exposure of selenium containing minerals to air and water can mobilize selenium by forming soluble ions such as selenate (SeO42−) and selenite (SeO32−). Solubility increases as the pH decreases. Mining activities, such as the refining of metal sulfide ores is one source of mobilized selenium. Selenium is also found in coal in small amounts but when coal-bearing strata are exposed to air and water during the mining process the selenium is mobilized and forms contaminated leachate, which often becomes a source of pollution to nearby surface waters. Coal and other fossil fuel combustion also release selenium to the environment. Well fertilized agricultural soil has about 400 mg/ton of selenium since the element is present in phosphate fertilizer.
Elemental forms of selenium include various valence or oxidation states. This speciation can be important because the differing valence states can have different chemical properties. Common selenium valence states include hexavalent selenium and tetravalent selenium. Also, elemental forms include metals in various valence states. This speciation can be important because the differing valence states can have contrasting physiological effects.
Selenium can be toxic in large amounts, and selenium is often measured in wastewater to ensure that it is present within acceptable concentrations. There are currently few selective, sensitive, specific, and inexpensive assays for metals and/or metalloids, inclusive of selenium, and various species thereof. Additionally, there are currently few assays that differentiate metal and/or metalloid species, for instance selenium species, by oxidation state. Further, the current technology cannot provide real time detection of selenium quickly. For instance, existing methodology such as ICP-MS is effective at measurement, but do not have acceptable throughput and turn-around time to allow responses to mitigate waste stream contamination to occur in an acceptable time frame.
There remains a need for new and improved sensors and arrays of sensors for measuring metals and/or metalloids.